This invention relates generally to a radio frequency identification (RFID) tag, and more particularly, to a RFID tag on a single layer substrate comprising a semiconductor integrated circuit RFID tag device and antenna circuit.
A radio frequency identification (RFID) tag is a device that stores identification information and sends back this identification information, and may also include other information, when the device is powered-up by a radio frequency (RF) signal. RFID tags utilize radio frequencies that have much better penetration characteristics to material than do optical signals, and will work under more hostile environmental conditions than bar code labels. Therefore, the RFID tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tags are used in conjunction with a radio frequency tag reader (interrogator) which transmits RF signals and receives data signals from the RFID tag. These RFID tags may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like. More applications are becoming commercially feasible as the cost of the RFID tags decrease.
The passive RFID tag has no internal power source, rather it uses the incoming RF signal as a power source. Once the RFID tag is activated, it sends stored information to the interrogator. The RFID tag transmits this stored information to the reader-interrogator by modulating the amplitude of the RF carrier signal from the reader by detuning a resonant circuit of the RFID tag that is initially tuned to the RF carrier signal (de-Qing or loading, for example by resistive loading, of the resonant circuit in the RFID tag may also be used to modulate the amplitude of the RF carrier signal of the reader-interrogator). The resonant circuit of the RFID tag may be, for example, a parallel connected inductor and capacitor which is used as an antenna and is resonant (tuned) to the frequency of the RF carrier signal of the interrogator. A semiconductor integrated circuit is connected to the parallel resonant antenna circuit and comprises an RF to direct current (DC) converter, a modulation circuit to send the stored information to the reader-interrogator, a logic circuit which stores coded information, a memory array that stores digitized information, and controller logic that controls the overall functionality of the RFID tag.
The inductor of the parallel resonant antenna circuit generally may be formed into a coil using wire or printed circuit conductors positioned on one surface of a planar dielectric (electrically insulated) substrate with connections being made between this coil and the RFID tag device semiconductor integrated circuit. These connections generally require the use of both sides of the planar substrate and increase the cost and complexity in manufacturing the RFID tag and decreases reliability thereof. Therefore, what is needed is a more reliable, simpler, lower cost, and easier to manufacture RFID tag.
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing an RFID tag comprising a semiconductor integrated circuit (RFID tag device) connected to an inductor coil of a parallel resonant circuit antenna and a jumper to one end of the parallel resonant circuit antenna, all being on one side of a dielectric substrate. The inductor coil may resonate with a discrete capacitor connected to the inductor coil, or a capacitor that is part of and internal to the semiconductor integrated circuit RFID tag device. A series resonant circuit antenna is also contemplated and within the scope of the invention.
In an embodiment of the invention, a RFID tag comprises a dielectric (electrically non-conductive and transparent to radio frequency signals) substrate having an antenna formed as an inductor by using electrically conductive material on only one side of the substrate. The substrate may be, for example but not limited to; PET, mylar, paper, plastic, kapton, ceramic, silicon, polyimide, polyvinylchloride (PVC), etc., and combinations thereof. A RFID tag device semiconductor integrated circuit die is attached to the substrate on the same side as the antenna inductor and is electrically connected thereto. The antenna inductor may be a spiral coil having two ends, an inner and an outer end. The inner end is connected to an inside spiral turn of the spiral coil and the outer end is connected to an outside spiral turn of the spiral coil. Generally, the semiconductor integrated circuit die is located on the inside of the spiral coil and is easily connected to the inner end of the spiral coil, since the inner end and the die may be in close proximity.
Connection to the RFID tag device semiconductor integrated circuit die may be by wire bonding, flipchip (C4), etc., or any combination thereof. The dielectric substrate may also have other connection pads that may be used for testing and/or programming the RFID tag. The semiconductor integrated die may be attached directly to the surface of the substrate, and the coil (and the other connection pads) may be formed on the same surface by printing, etching, hot stamping and the like. This type of coil construction allows better conductance (inverse of resistance) which results in a higher Q of the coil. The coil material is electrically conductive and may be, for example but not limited to; metal such as copper, aluminum, gold, plated metal, electrically conductive organic and inorganic materials, etc.
The outer end of the spiral coil is connected to the RFID tag device integrated circuit die with a jumper that is also on the same side of the substrate as the antenna coil and integrated circuit die. The jumper goes over the spiral coil from the outer end to a bond pad of the integrated circuit die in the case of wire bonding where the back of the die is attached to the surface of the substrate and the connection pads of the integrated circuit die are facing away from the surface of the substrate.
The jumper may be a bond wire connected by thermal compression bonding, ball bonding and the like. The jumper may also be any type of conductor that is cut, etched, deposited and the like which can be insulated from the inductor coil when passing thereover.
When using wire bonding, all connections to the integrated circuit die are made by bond wires, i.e., inner and outer ends of the coil, programming and test pads, etc. After the bond wires are installed, an encapsulation (glop top) cover may be used to protect the integrated circuit die and bond wires. This form of construction is easy to manufacture and low in cost. Intermediate pads may be used between the coil turns to reduce the length of the bond wire going from the outer end of the coil to the bond pad of the integrated circuit die. The inductor coil may be coated with a insulating coating so as to make the RFID tag completely insulated. Thus, a low cost xe2x80x9cchip-on-tagxe2x80x9d is created with no further processing required.
The semiconductor integrated circuit die may also be attached to connection pads on the substrate by using flipchip or C4 connections wherein xe2x80x9csolder ball bumpsxe2x80x9d on bond pads of the die attach to the substrate pads and all other connections to these pads are by printed circuit conductors and a bond wire (jumper across coil turns). A thin insulating layer such as polyimide may be used between the coil jumper and the inductor coil turns to prevent shorting of the coil, however, once the encapsulation (glop top) is in place and has cured, no movement of the jumper bond wire can occur.
In another embodiment of the invention, a flipchip die straddles the inductor coil turns so that one solder ball bump of the flipchip die connects to the outer end of the inductor coil and another solder ball bump of the flipchip die connects to the inner end of the inductor coil. A conductive trace may also be provided within the flipchip die that is a conductive circuit within the die that may serve as a jumper over the turns of the inductor coil. The conductive trace may be adapted to connect an external capacitor in parallel with the inductor coil. The external capacitor may be located inside or outside of the inductor coil turns and may be on the same side of the substrate as the inductor coil and die.
In still another embodiment, a die may be attached over a portion of the inductor coil turns with an insulating layer of material therebetween. The insulating layer may be a B-staged kapton or epoxy that may be cured so as to attach the die to the substrate. Mylar, mica, plastic, teflon, kapton, polyimide and the like may be used as an insulating layer that is attached to the substrate and the die. The die has bond pads thereon and wire bonding (bond wires) may be used to connect the die bond pads to the inner and outer ends of the inductor coil turns. The bond wires may be used as jumpers over the inductor coil turns and further may allow the inductor coil turns to remain at full width while passing under the die. By allowing the inductor coil turns to remain at full width, the coil turns have a lower resistance and thus yield a higher quality factor (higher Q). A conductive trace may also be provided within the die that is a conductive circuit through the die that may be adapted to connect an external capacitor in parallel with the inductor coil. The external capacitor may be located inside or outside of the inductor coil turns and may be on the same side of the substrate as the inductor coil and die.
Features and advantages of the invention will be apparent from the following description of presently preferred embodiments, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.